A low dropout or LDO voltage regulator is an electronic circuit that is designed to provide a stable output voltage regardless of input voltage variations and load impedance. An LDO voltage regulator is able to maintain output regulation even for a relatively small difference between the input voltage and the output voltage. When regulating the voltage from a battery, an LDO voltage regulator can maintain a steady output voltage for input voltages ranging from high battery voltages down to voltage levels just above the output voltage.
In a linear LDO voltage regulator, a transistor pass element connected between the input and the output is operated in its linear region when the input battery voltage is close to the targeted output voltage. The resistance and thus the voltage drop across the pass element are varied to maintain the desired output voltage. The dropout voltage determines the lowest usable input voltage and is related to the on resistance of the pass element.
The pass element is generally controlled by an error amplifier based on feedback from the output voltage. The error amplifier lowers the resistance of the pass element if the output voltage drops and raises the resistance of the pass element if the output voltage rises. Both p-channel metal oxide semiconductor (PMOS) and n-channel metal oxide semiconductor (NMOS) transistors may be used as pass elements in an LDO voltage regulator. An NMOS transistor has relatively high conductivity, and the error amplifier in an LDO voltage regulator can drive the gate of an NMOS pass element with a voltage just above the output voltage. Because NMOS transistors are much smaller than PMOS transistors, they may be advantageous as a pass element in an LDO voltage regulator. In order to generate enough gate-to-source voltage for the NMOS pass element when the input voltage gets close to the output voltage, the error amplifier is typically powered by a charge pump. However, a charge pump running at high frequencies consumes considerable power to achieve a low amount of ripple on the output voltage. In addition, the charge pump and associated oscillator generates undesirable electromagnetic interference (EMI). Thus, although the small relative size of an NMOS transistor pass element may reduce the size of an LDO voltage regulator, running a charge pump constantly in the LDO voltage regulator is not without disadvantages.